Refrigeration lubricant composition

ABSTRACT

A refrigeration lubricant composition comprises an ester obtained from a mixed alcohol and a mixed carboxylic acid as a main component. The mixed alcohol comprises 65 to 99.95 mol % of pentaerythritol and 0.05 to 35 mol % of dipentaerythritol, the mixed carboxylic acid comprises 25 to 55 mol % of monocarboxylic acid having 5 to 8 carbon atoms and 45 to 75 mol % of isononanoic acid, and the composition has a hydroxyl value of 5.0 mgKOH/g or less and an acid value of 0.05 mgKOH/g or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a refrigeration lubricant composition.More specifically, the present invention relates to a refrigerationlubricant composition that is used for a chlorine-free hydrofluorocarbonrefrigerant, wherein the composition comprises a polyol ester as themain component and has excellent low-temperature stability, lubricity,stability under coexistence with a chlorine-free hydrofluorocarbonrefrigerant, and has excellent compatibility with chlorine-freehydrofluorocarbon refrigerants. The present invention also relates to arefrigerant working fluid containing the refrigeration lubricantcomposition and a refrigerating apparatus including the refrigerantworking fluid.

2. Description of the Related Art

Conventionally, refrigerants containing chlorofluorocarbon have beenused for air conditioning equipment such as room air conditioners andpackaged air conditioners, low temperature apparatuses such asrefrigerator-freezers for home use, industrial refrigerators, andautomotive air conditioners such as hybrid cars and electric cars.However, in recent years, the replacement of such chlorofluorocarbonrefrigerants with chlorine-free hydrofluorocarbon refrigerants such asthose containing 1,1,1,2-tetrafluoroethane (R-134a), pentafluoroethane(R-125), difluoromethane (R-32), and mixtures thereof has been promotedbecause of problems such as the depletion of the ozone layer.Consequently, a variety of refrigeration oils containing a polyol ester,which has good compatibility with chlorine-free hydrofluorocarbonrefrigerants, as a base stock have been proposed.

In addition to the above-described compatibility with chlorine-freehydrofluorocarbon refrigerants, a variety of properties such aslubricity, thermal stability, hydrolytic stability, and low-temperaturefluidity are required for refrigerating machine oils in order to ensurethe stability of the above-described equipment. Among these, in view ofhydrolytic stability and compatibility with chlorine-freehydrofluorocarbon refrigerants, hindered esters having excellent thermalresistance have been put to practical use. These hindered esters arederived from a carboxylic acid having a methyl branched chain or anethyl branched chain in the α position or the β position andpentaerythritol. For example, Japanese Laid-Open Patent Publication No.10-8084 discloses refrigeration oils containing an ester obtained frompentaerythritol and a mixed fatty acid of 2-ethylhexanoic acid and3,5,5-trimethylhexanoic acid as the main component and discloses thatsuch refrigeration oils have good stability at high temperatures.Moreover, Japanese Laid-Open Patent-Publication No. 5-209181 disclosesesters obtained from pentaerythritol, dipentaerythritol, andtripentaerythritol and discloses the viscosity and the compatibilitywith fluorocarbons of such esters. Furthermore, Japanese Laid-OpenPatent Publication No. 6-330061 discloses refrigeration oils containingan ester obtained from pentaerythritol and a mixed fatty acid of alinear or branched fatty acid having 6 to 8 carbon atoms and3,5,5-trimethylhexanoic acid as the main component. The publicationdiscloses that such refrigeration oils have an excellent compatibilitywith refrigerants and have improved electric insulation properties.Furthermore, in order to improve lubricity and compatibility withrefrigerants, Japanese Laid-Open Patent Publication No. 10-158215discloses esters obtained from a fatty acid mixture and a polyhydricalcohol, wherein the fatty acid mixture includes 3,5,5-trimethylhexanoicacid and a saturated fatty acid other than 3,5,5-trimethylhexanoic acid,wherein the saturated fatty acid has an alkyl group as a side chain andhas a total of 9 carbon atoms.

In a refrigerating cycle, a part of the refrigeration lubricantgenerally circulates through the cycle along with a refrigerant, andthus the refrigeration lubricant is exposed to a high temperature regionand a low temperature region. As for high thermal resistance required ina compressor, which operates in the high temperature region, theabove-described hindered esters, in particular, the mixed esters of thepatent publications noted above satisfy the performance requirements. Onthe other hand, however, a part of the refrigeration lubricant ejectedfrom the compressor may stagnate in the low temperature region. Inparticular, if the lubricant stagnates in the low temperature region fora long period of time, then it is crystallized and the amount of thecirculating refrigerant in the refrigerating cycle is reduced, which maylead to problems such as poor refrigeration. Therefore, it is criticalto the reliability of refrigerating apparatuses to develop arefrigeration lubricant higher stability in which no precipitationoccurs over a long period of time even at low temperatures. However, themixed esters obtained from a combination of a polyhydric alcohol and aspecific carboxylic acid as described in the patent publications notedabove have not been sufficiently examined for their ability ofmaintaining low-temperature stability over a long period of time. Thus,it is not possible to prevent crystallization from occurring in the lowtemperature region, and thus these esters lack long-term stability.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a refrigerationlubricant composition that does not easily crystallize at lowtemperatures, in other words, a refrigeration lubricant composition thathas excellent low-temperature stability. It is another object of thepresent invention to provide a refrigeration lubricant compositionhaving high viscosity, excellent compatibility with chlorine-freehydrofluorocarbon refrigerants, and excellent long-term stability in alow temperature region of the refrigerating machine.

The inventors of the present invention carefully examined thelow-temperature stability of refrigeration lubricant compositions andthe compatibility thereof with chlorine-free hydrofluorocarbonrefrigerants and performed molecular design by combining a variety ofpolyhydric alcohols with mixed carboxylic acids containing a variety ofcarboxylic acids having different structures. As a result, the inventorsfound that the above-described problems can be solved by an esterobtained by reacting a mixed alcohol containing pentaerythritol anddipentaerythritol in a specific ratio with a mixed carboxylic acidcontaining a specific amount of isononanoic acid, and thus the presentinvention was accomplished. Furthermore, in performing the moleculardesign of the above-described ester, the inventors of the presentinvention found that an ester having excellent long-term low-temperaturestability and having excellent compatibility with chlorine-freehydrofluorocarbon refrigerants can be obtained in the following case: amixed carboxylic acid is constituted by considering the length of a mainchain (regardless of whether a linear carboxylic acid or a branchedcarboxylic acid) and the length of the longest side chain of branchedcarboxylic acids; and the carboxylic acids in the mixed carboxylic acidand the alcohols in the mixed alcohol satisfy a certain quantitativerelation.

The refrigeration lubricant composition of the present inventioncomprises a mixed ester obtained from a mixed alcohol and a mixedcarboxylic acid as a main component; and the mixed alcohol includes 65to 99.95 mol % of pentaerythritol and 0.05 to 35 mol % ofdipentaerythritol, the mixed carboxylic acid includes 25 to 55 mol % ofmonocarboxylic acid having 5 to 8 carbon atoms and 45 to 75 mol % ofisononanoic acid, and the composition has a hydroxyl value of 5.0mgKOH/g or less and an acid value of 0.05 mgKOH/g or less.

In a preferred embodiment, the mixed ester is obtained by a reaction ofthe mixed alcohol and the mixed carboxylic acid, wherein the mixedcarboxylic acid and the mixed alcohol comprise carboxylic acids andalcohols, respectively, which satisfy the following relation:$\begin{matrix}{1.09 \leqq \frac{\left\{ {{\sum\quad\left( {A \times B} \right)} - \left( {C \times D} \right)} \right\}}{E} \leqq 1.33} & (1)\end{matrix}$

-   -   A: number of carbon atoms in main chain of a carboxylic acid in        the mixed carboxylic acid    -   B: molar fraction of the carboxylic acid of item “A”    -   C: number of carbon atoms in the longest side chain of        carboxylic acids in the mixed carboxylic acid    -   D: molar fraction of carboxylic acid having the longest side        chain    -   E: average number of hydroxyl groups of each alcohol in the        mixed alcohol

In a preferred embodiment, a kinematic viscosity of the composition at40° C. is 30 to 150 mm²/s.

In a preferred embodiment, the isononanoic acid contains 88.50 to 99.95mol % of 3,5,5-trimethylhexanoic acid.

In a preferred embodiment, the composition is used for a chlorine-freehydrofluorocarbon refrigerant.

The refrigerant working fluid of the present invention comprises therefrigeration lubricant composition and a chlorine-freehydrofluorocarbon refrigerant.

The refrigerant compression type refrigerating apparatus of the presentinvention comprises a compressor, a condenser, an expansion mechanism,an evaporator, and the above mentioned working fluid.

The refrigeration lubricant composition of the present invention doesnot cause precipitation of crystals over a long period of time in thelow temperature region, and thus it has excellent low-temperaturestability. Moreover, it also has good lubricity, stability undercoexistence with a chlorine-free hydrofluorocarbon refrigerant(evaluated by sealed tube test), and compatibility with chlorine-freehydrofluorocarbon refrigerants, which are required for refrigerationlubricant compositions. Therefore, the refrigeration lubricantcomposition of the present invention is useful for a lubricant forrefrigerating machines that employ a chlorine-free hydrofluorocarbonrefrigerant or as a refrigerant working fluid in which it is mixed witha chlorine-free hydrofluorocarbon refrigerant.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The inventors of the present invention found that an ester obtained byreacting a mixed alcohol containing pentaerythritol anddipentaerythritol in a specific ratio with a mixed carboxylic acidcontaining a specific amount of isononanoic acid and, if necessary,containing a carboxylic acid other than isononanoic acid has excellentlow-temperature stability.

Hereinafter, a mixed ester contained in the composition of the presentinvention, a refrigeration lubricant composition containing the mixedester, a refrigerant working fluid containing the composition, and arefrigerant compression type refrigerating apparatus employing therefrigerant working fluid will be described;

(Mixed Ester)

The mixed ester contained in the refrigeration lubricant composition ofthe present invention is obtained by reacting a mixed alcohol and mixedcarboxylic acids, wherein the mixed alcohol comprises 65 to 99.95 mol %of pentaerythritol and 0.05 to 35 mol % of dipentaerythritol, andwherein the mixed carboxylic acid comprises 25 to 55 mol % ofmonocarboxylic acid having 5 to 8 carbon atoms and 45 to 75 mol % ofisononanoic acid.

In this reaction, in order to obtain a mixed ester having excellentlow-temperature stability over an even longer period of time, it isparticularly preferable that the reaction between the mixed alcohol andthe mixed carboxylic acid that are employed satisfy the followingrelation: $\begin{matrix}{1.09 \leqq \frac{\left\{ {{\sum\quad\left( {A \times B} \right)} - \left( {C \times D} \right)} \right\}}{E} \leqq 1.33} & (1)\end{matrix}$

-   -   A: number of carbon atoms in main chain of a carboxylic acid in        the mixed carboxylic acid    -   B: molar fraction of the carboxylic acid of item “A”    -   C: number of carbon atoms in the longest side chain of        carboxylic acids in the mixed carboxylic acid    -   D: molar fraction of carboxylic acid having the longest side        chain    -   E: average number of hydroxyl groups of each alcohol in the        mixed alcohol

The inventors of the present invention found the following: when a mixedester contains branched esters that are derived from carboxylic acidshaving a side chain or side chains with different carbon numbers, thecarboxylic acid having the longest side chain contributes to the lowtemperature stability of the mixed ester more effectively thancarboxylic acids having a shorter branched chain does. The inventorsconducted molecular design of esters and found that when the above-notedformula was satisfied, a mixed ester having excellent low-temperaturestability over a longer period of time could be obtained.

“Number of carbon atoms in main chain of a carboxylic acid in the mixedcarboxylic acid” defined in item A of the above-noted relation refers tothe number of carbon atoms of the longest chain of the carboxylic acidand excluding carbon atoms in side chains. For example, in the case ofoctylic acid, which is a linear carboxylic acid having 8 carbon atoms,the number of carbon atoms in the main chain of the carboxylic acid is8. In the case of 2-ethylhexanoic acid, which is a branched carboxylicacid having 8 carbon atoms in total, the number of carbon atoms in themain chain of the carboxylic acid is 6, which is obtained by subtracting2 (i.e., the number of carbon atoms in the branched ethyl group) fromthe total number of carbon atoms 8 in the carboxylic acid (i.e.,2-ethylhexanoic acid).

“Molar fraction of carboxylic acid of item A” defined in item B of theabove-noted relation refers to a value of the molar amount of thecarboxylic acid having the number of carbon atoms in main chain definedin item A when the total molar amount of the mixed carboxylic acid is 1.

“Number of carbon atoms in the longest side chain of carboxylic acids inthe mixed carboxylic acid” defined in item C of the above-noted relationrefers to the number of carbon atoms in the longest side chain ofcarboxylic acids having the longest side chain in the branchedcarboxylic acids that are contained in the mixed carboxylic acid. Forexample, in the case of a mixed carboxylic acid of 2-ethylhexanoic acidand 3,5,5-trimethylhexanoic acid, the number of carbon atoms in thelongest side chain is 2, which is the number of carbon atoms from theethyl group of 2-ethylhexanoic acid.

“Molar fraction of carboxylic acid having the longest branched chain”defined in item D of the above-noted relation refers to a value of themolar amount of the carboxylic acid having the longest side chain whenthe total molar amount of the mixed carboxylic acid is 1.

“Average number of hydroxyl groups of each alcohol in the mixed alcohol”defined in item E of the above-noted relation refers to the sum ofvalues, each of which is obtained by multiplying the number of hydroxylgroups of each alcohol contained in a mixed alcohol by the molarfraction of each alcohol, wherein molar fraction is a value of the molaramount of that alcohol when the total molar amount of the mixed alcoholis 1. For example, in the case of a mixed alcohol made of 75 mol % ofpentaerythritol (the number of hydroxyl groups is 4) and 25 mol % ofdipentaerythritol (the number of hydroxyl groups is 6), the averagenumber of hydroxyl groups in the mixed alcohol is 4×0.75+6×0.25=4.5.

The mixed alcohol that serves as a raw material of the above-describedmixed ester is made of pentaerythritol and dipentaerythritol, asdescribed above. The pentaerythritol content in the mixed alcohol is 65to 99.95 mol %, preferably 70 to 99.95 mol %, and more preferably 75 to99.95 mol %. On the other hand, the dipentaerythritol content in themixed alcohol is 0.05 to 35 mol %, preferably 0.05 to 30 mol %, and morepreferably 0.05 to 25 mol %. When the pentaerythritol content in themixed alcohol is 65 mol % or more,. the resultant ester has high levelsof viscosity, low-temperature fluidity, and compatibility withchlorine-free hydrofluorocarbon refrigerants, and when it is 99.95 mol %or less, then the resultant ester has long-term low-temperaturestability. On the other hand, when the dipentaerythritol content in themixed alcohol is 0.05 mol % or more, then the resultant ester haslong-term low-temperature stability, and when it is 35 mol % or less,then the resultant ester has suppressed deterioration in compatibilitywith chlorine-free hydrofluorocarbon refrigerants and suppressedincrease in viscosity.

The mixed carboxylic acid that serves as a raw material of theabove-described mixed ester is made of a monocarboxylic acid having 5 to8 carbon atoms and an isononanoic acid. The monocarboxylic acid having 5to 8 carbon atoms is contained in the mixed carboxylic acid in a ratioof 25 to 55 mol %, preferably 30 to 55 mol %, more preferably 33 to 55mol %, and even more preferably 35 to 50 mol %. The isononanoic acid iscontained in the mixed carboxylic acid in a ratio of 45 to 75 mol %,preferably 45 to 70 mol %, more preferably 45 to 67 mol %, and even morepreferably 50 to 65 mol %. When the isononanoic acid content in themixed carboxylic acid is 45 to 75 mol %, then a mixed ester that hasexcellent long-term low-temperature stability and compatibility withchlorine-free hydrofluorocarbon refrigerants can be obtained.

Examples of the monocarboxylic acid having 5 to 8 carbon atoms includepentanoic acid, 2-methylbutanoic acid, 3-methylbutanoic acid, hexanoicacid, 2-methylpentanoic acid, 3-methylpentanoic acid, 4-methylpentanoicacid, 2-ethylbutanoic acid, 3-ethylbutanoic acid, heptanoic acid,2-methylhexanoic acid, 3-methylhexanoic acid, 4-methylhexanoic acid,5-methylhexanoic acid, 2,2-dimethylpentanoic acid, 2-ethylpentanoicacid, 3-ethylpentanoic acid, 2-ethylhexanoic acid, 2-methylheptanoicacid, 3-methylheptanoic acid, 4-methylheptanoic acid, 5-methylheptanoicacid, 6-methylheptanoic acid, 2,2-dimethylhexanoic acid, and3,5-dimethylhexanoic acid. 2-ethylhexanoic acid is preferably used sincethe obtained lubricant has good lubricity and hydrolytic stability andis difficult to corrode metals. The above-noted monocarboxylic acidshaving 5 to 8 carbon atoms may be used either alone or in combination oftwo or more thereof.

Examples of the isononanoic acid include 2,5,5-trimethylhexanoic acid,3,5,5-trimethylhexanoic acid, 4,5,5-trimethylhexanoic acid,2,2,4,4-tetramethylpentanoic acid, 2-ethyl-4,4-dimethylpentanoic acid,6,6-dimethylheptanoic acid, 4-ethyl-2-methylhexanoic acid,2-methyloctanoic acid, and 2-ethylheptanoic acid.2,5,5-Trimethylhexanoic acid, 3,5,5-trimethylhexanoic acid,4,5,5-trimethylhexanoic acid, and 6,6-dimethylheptanoic acid arepreferable, and 3,5,5-trimethylhexanoic acid is more preferable. Theabove-noted isononanoic acids may be used either alone or in combinationof two or more thereof.

When the isononanoic acid contains 3,5,5-trimethylhexanoic acid, theresultant lubricant composition has excellent compatibility withfluorocarbons and has long-term stability in the low temperature region.Thus, it is preferable that the isononanoic acid contains3,5,5-trimethylhexanoic acid in a ratio of 88.50 to 99.95 mol %, morepreferably 90.0 to 99.5 mol %, and even more preferably 92.0 to 99.0 mol%.

By the combination of a polyhydric alcohol that satisfies theabove-described mixing amounts of pentaerythritol and dipentaerythritolwith an isononanoic acid having the above-described branch structure anda monocarboxylic acid having 5 to 8 carbon atoms, an ester that hasexcellent low-temperature stability and compatibility with chlorine-freehydrofluorocarbon refrigerants can be obtained. More preferably, forexample, it is possible to obtain the value of the numerator of relation(1) from a carboxylic acid selected from the above-described range,determine the average number of hydroxyl groups of the denominator thatsatisfies the numerical range of relation (1), and determine the amountsof pentaerythritol and dipentaerythritol within the range of theabove-described mixing ratio of pentaerythritol and dipentaerythritolsuch that the determined average number of hydroxyl groups is achieved.By designing in this manner, a mixed ester that has excellentlow-temperature stability over a longer period of time, lubricity,stability under coexistence with a chlorine-free hydrofluorocarbonrefrigerant, and compatibility with chlorine-free hydrofluorocarbonrefrigerants can be obtained.

The mixed ester used in the present invention is obtained by ordinaryesterification reaction or transesterification. The ratio of theabove-described mixed alcohol and the above-described mixed carboxylicacid is determined as appropriate such that the obtained mixed ester hasa hydroxyl value of 5.0 mgKOH/g or less and an acid value of 0.05mgKOH/g or less.

Specifically, the mixed ester used in the present invention is obtainedin the following manner. First, a mixed carboxylic acid is mixed with amixed alcohol such that the mixed carboxylic acid is 1.0 to 1.5equivalents, preferably 1.05 to 1.3 equivalents, with respect to 1equivalent of hydroxyl group in the mixed alcohol, and then a catalystis added thereto, if necessary. This mixture is reacted for 3 to 15hours at 220 to 260° C. under a nitrogen, and at the point when thehydroxyl value becomes 3.0 mgKOH/g or less, excess carboxylic acid isremoved under a reduced pressure. Then, after neutralization with analkali, operations such as adsorption treatment using activated clay,acid clay, and a synthesized adsorbent, and steaming are performedeither alone or in combination.

Refrigeration Lubricant Composition

The refrigeration lubricant composition of the present inventioncontains the above-described mixed ester as the main component and maycontain, for example, an ester other than the mixed ester and anadditive, if necessary. The specific amount of the “main component” is50 wt % or more, preferably 70 wt % or more, and more preferably 90 wt %or more based on the entire refrigeration lubricant composition.

Examples of the ester other than the above-described mixed ester includeesters obtained from at least one alcohol and the above-describedmonocarboxylic acid having 5 to 9 carbon atoms, wherein the alcohol isselected from the group consisting of pentaerythritol,dipentaerythritol, and neopentyl polyol having 2 to 8 hydroxyl groupsand having 5 to 15 carbon atoms (e.g., neopentyl glycol,2,2-diethyl-1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol,trimethylolethane, triethylolethane, trimethylolpropane,tripentaerythritol, and bispentaerythritol).

Examples of the additive include a phenol antioxidant, a metaldeactivator such as benzotriazole, thiadiazole, and dithiocarbamate, anacid scavenger such as epoxy compounds and carbonimides, and aphosphorous extreme pressure agent. The additive is contained in anyratio.

The refrigeration lubricant composition of the present invention has ahydroxyl value of 5.0 mgKOH/g or less, and preferably 3.0 mgKOH/g orless, more preferably 2.0 mgKOH/g or less, and even more preferably 1.0mgKOH/g or less. When the hydroxyl value is 5.0 mgKOH/g or less, thenthe composition has sufficient electric insulation property withoutdeteriorating the volume resistivity of the composition. Therefore, inequipment in which the above-described composition is employed, there isno possibility of an adverse effect such as dissolving a sealingmaterial that is made of an organic material. Also, there is nopossibility of an adverse effect on additives that may be contained inthe composition.

The refrigeration lubricant composition of the present invention has anacid value of 0.05 mgKOH/g or less, and preferably it may-have an acidvalue of 0.03 mgKOH/g or less and more preferably 0.01 mgKOH/g or less.When it has an acid value of 0.05 mgKOH/g or less, then the compositionis less likely to corrode metals and has good hydrolytic stability.

There is no particular limitation regarding the kinematic viscosity ofthe refrigeration lubricant composition of the present invention. Inview of lubricity, compatibility with fluorocarbons, startability ofrefrigerating machines, and energy saving, it is preferable that thekinematic viscosity at 40° C. is 30 to 150 mm²/s. In particular, whenthe above-described lubricant composition is used for air-conditioningequipment such as room air conditioners and packaged air conditioners;low temperature apparatuses; industrial refrigerators; and compressorsin automotive air conditioners such as hybrid cars and electric cars, 55to 140 mm²/s is preferable, and 60 to 130 mm²/s is more preferable, inview of the operating efficiency thereof.

The base stock of the refrigeration lubricant of the present inventionhas excellent long-term stability even in the low temperature region.Moreover, it has excellent compatibility with fluorocarbons and thermalresistance, and when used for air conditioning equipment and compressorsused for automotive air conditioners, it can improve operatingefficiency compared with the conventional refrigeration lubricants. Thebase stock of the refrigeration lubricant of the present invention hasexcellent compatibility with, in particular, chlorine-freehydrofluorocarbons, and thus it is useful for a lubricant forchlorine-free hydrofluorocarbon refrigerant.

Refrigerant Working Fluid

The refrigerant working fluid of the present invention is made of theabove-described refrigeration lubricant composition and a chlorine-freehydrofluorocarbon refrigerant. There is no particular limitationregarding the amounts of the refrigeration lubricant composition and thechlorine-free hydrofluorocarbon refrigerant, but a weight ratio of thelubricant composition and the chlorine-free hydrofluorocarbonrefrigerant is in the range of 10:90 to 90:10 preferably. If the amountsof the chlorine-free hydrofluorocarbon refrigerant is higher than theabove-described range, then the viscosity of the resultant refrigerantworking fluid is reduced, which may lead to poor lubrication. If it islower than the above-described range, then when the obtained workingfluid is used for refrigerating apparatuses, refrigerating efficiencymay deteriorate.

Examples of the chlorine-free hydrofluorocarbon refrigerant include1,1,1,2-tetrafluoroethane (R-134a), pentafluoroethane (R-125),difluoroethane (R-32), trifluoroethane (R-23), 1,1,2,2-tetrafluoroethane(R-134), 1,1,1-trifluoroethane (R-143a), and 1,1-difluoroethane(R-152a). These refrigerants may be used either alone or as mixedrefrigerants of two or more thereof.

The above-described mixed refrigerants are commercially available, and,for example, R-407C (R-134a/R-125/R-32=52/25/23 wt %), R-410A(R-125/R-32=50/50 wt %), R-404A (R-125/R-143a/R-134a=44/52/4 wt %),R-407E (R-134a/R-125/R-32=60/15/25 wt %), and R-410B (R-32/R-125=45/55wt %) are used. In particular, mixed refrigerants containing at leastone of R-134a and R-32 are preferable.

(Refrigerant Compression Type Refrigerating Apparatus)

The refrigerant compression type refrigerating apparatus of the presentinvention is provided with a compressor, a condenser, an expansionmechanism, and an evaporator and is configured such that theabove-described refrigerant working fluid, which is the refrigerant inthe refrigerating apparatus, circulates through these components. Thisrefrigerating apparatus may further include a drier. Examples of such arefrigerating apparatus include air conditioning equipment such as roomair conditioners and packaged air conditioners; low temperatureapparatuses; industrial refrigerators; and automotive air conditionerssuch as hybrid cars and electric cars.

EXAMPLES

Hereinafter, the present invention will be described more specificallyby way of examples.

The test method of esters produced in the examples and the comparativeexamples will be described below:

-   -   <Kinematic viscosity and viscosity index> The kinematic        viscosity is measured with a Cannon-Fenske viscometer at 40° C.        and 100° C. according to JIS K-2283, and the viscosity index is        calculated from the resultant values.    -   <Acid value> The acid value is measured according to JIS C-2101.    -   <Hydroxyl value> The hydroxyl value is measured according to JIS        K-0070.    -   <Color number (APHA)> The color number (APHA) is measured        according to JOCS 2.2.1.4-1996.    -   <Volume resistivity> The volume resistivity (TΩ·m) at 25° C. is        measured according to JIS C-2101.    -   <Pour point> The pour point is measured according to JIS K-2269.    -   <Long-term low-temperature test> First, 400 g of a sample ester,        the moisture content of which was adjusted to 100 ppm or less,        is placed in a square can made of steel and allowed to stand for        1000 hours in a low temperature storage at −20° C., and then        checked visually to determine whether or not crystals are        precipitated.    -   <Two-phase separation temperature> First, 0.6 g of a sample        ester and 2.4 g of the refrigerants R-134a and R-407C were        enclosed in a thick PYREX (registered trademark) tube (entire        length of 300 mm, outer diameter of 10 mm, and inner diameter of        6 mm) cooled in an ethanol bath containing dry ice and warmed or        cooled at a rate of 1° C./min. Then the two-phase separation        temperatures at a high temperature and a low temperature were        measured visually within a temperature range from −70° C. to        +80° C.    -   <Sealed tube test> First, 10 g of a sample ester, the moisuture        content of which has been adjusted to 200 ppm or less, 5 g of        the fluorocarbon R-410A, and one each of iron, copper, and        aluminum metal pieces having a length of 10 mm are put in a        glass tube and the glass tube is sealed. This is heated at        175° C. for 14 days, and then the acid value and the color        number (APHA) for the fluorocarbon-containing sample from which        the metal pieces were removed are measured.    -   <Falex friction test> The Falex friction test was performed        according to ASTM D-2670 while blowing R-134a into a sample        ester at a rate of 150 mL/min. The temperature of the sample is        set at 100° C., and an operation under a load of 250 pound is        carried out for 2 hours after a trial operation under a load of        150 pound for one minutes, and the wear amount of the pin after        the end of the operation is measured.

Example 1 Preparation of Ester

First, the mixed alcohol and the mixed carboxylic acid shown in Table 1were placed in a one-liter four-necked flask provided with athermometer, a nitrogen inlet tube, a stirrer and a cooling tube suchthat the ratio of the hydroxyl group in the mixed alcohol and thecarboxyl group in the mixed carboxylic acid is 1:1.1 in the equivalentratio, and then reacted under a nitrogen at 220° C. at an atmosphericpressure while water generated by the reaction was removed bydistillation. During the reaction, the hydroxyl value of the reactionmixture was monitored, and the reaction was stopped at the point whenthe hydroxyl value became lower than 2.0 mgKOH/g. Then, stripping wasperformed under a reduced pressure of 1 to 5 kPa to remove unreactedcarboxylic acid for one hour. The resultant reaction mixture was washedwith an aqueous solution of potassium hydroxide. Washing was repeatedfive times so that the pH of the discharged water became neutral. Then,the resultant ester layer was dehydrated at 100° C. under a reducedpressure of 1 kPa, and acid clay and a silica-alumina adsorbent wereadded thereto such that each of them was contained 1.0 wt % of thetheoretical amount of an ester to be obtained for an adsorptiontreatment. The adsorption treatment temperature, pressure, andadsorption treatment time were 100° C., 1 kPa, and 3 hours,respectively. The mixture was filtrated, and thus an ester (this esteris referred to as ester A) was obtained. The kinematic viscosity at 40°C. and 100° C. and the viscosity index of the obtained ester A weremeasured according to the above-described method. Table 1 shows theresults.

Examples 2 to 6

Esters (i.e., esters B to F) were obtained in the same manner as inExample 1 except that the mixed alcohols and the mixed carboxylic acidsshown in Table 1 were used. The kinematic viscosity at 40° C. and 100°C. and the viscosity index of each ester were measured according to theabove-described method. Table 1 also shows the results.

Comparative Examples 1 to 5

Esters (i.e., esters G to K) were obtained in the same manner as inExample 1 except that the mixed alcohols and the mixed carboxylic acidsshown in Table 1 were used. The kinematic viscosity at 40° C. and 100°C. and the viscosity index of each ester were measured according to theabove-described method. Table 1 also shows the results. TABLE 1 MixedKinematic viscosity alcohol *1 Mixed carboxylic acid Value of (mm²/s)Viscosity (mol %) (mol %) relation (1) Ester 40° C. 100° C. index Ex. 1PE (69.5) Pentanoic acid (10) 1.11 A 106.9 11.14 87 diPE (30.5)2-Ethylhexanoic acid (39) 3,5,5-Trimethylhexanoic acid (51) 2 PE (81.4)2-Ethylhexanoic acid (48) 1.15 B 90.54 10.02 89 diPE (18.6)3,5,5-Trimethylhexanoic acid (50) 2,5,5-Trimethylhexanoic acid (2) 3 PE(88.2) 2-Methylhexanoic acid (5) 1.22 C 79.76 9.127 87 diPE (11.8)2-Ethylhexanoic acid (42) 3,5,5-Trimethylhexanoic acid (52)4,5,5-Trimethylhexanoic acid (1) 4 PE (97.2) 2-Ethylhexanoic acid (44)1.26 D 75.82 8.831 87 diPE (2.8) 3,5,5-Trimethylhexanoic acid (56) 5 PE(99.4) 2-Ethylhexanoic acid (38) 1.31 E 76.84 8.812 84 diPE (0.6)3,5,5-Trimethylhexanoic acid (61) 6,6-Dimethylheptanoic acid (1) 6 PE(99.5) 2-Ethylhexanoic acid (49) 1.25 F 68.12 8.142 84 diPE (0.5)3,5,5-Trimethylhexanoic acid (50) 2,2,4,4-Tetramethylpentanoic acid (1)Com. 1 PE (100) 2-Ethylhexanoic acid (31) 1.35 G 81.95 9.312 87 Ex.3,5,5-Trimethylhexanoic acid (69) 2 PE (100) 2-Ethylhexanoic acid (100)1.00 H 44.58 6.264 83 3 PE (100) 3,5,5-Trimethylhexanoic acid (100) 1.25I 114.2 11.35 83 4 diPE (100) 2-Ethylhexanoic acid (50) 0.83 J 243 19.1588 3,5,5-Trimethylhexanoic acid (50) 5 NPG (100) 2-Ethylhexanoic acid(100) 2.00 K 7.481 2.045 48*1 PE: Pentaerythritol, diPE: Dipentaerythritol

Example 7 Preparation of Base Stock of Lubricant

The above-described ester A was used as a base stock of a lubricant(this base stock is referred to as base stock 1). The kinematicviscosity at 40° C. and 100° C., viscosity index, acid value, hydroxylvalue, color number, volume resistivity, pour point, two-phaseseparation temperature, and Falex test pin friction amount of theobtained base stock 1 were measured according to the above-describedmethods. Furthermore, the long-term low-temperature test and the sealedtube test were performed. Table 2 shows the results.

Examples 8 to 14

Any one or more of the esters B to F, H, and K obtained in theabove-described examples and comparative examples were employed as shownin Table 2 to prepare base stocks, of a lubricant (i.e., base stocks 2to 8). The base stocks were tested in the same manner as in Example 1.Table 2 also shows the results.

Comparative Examples 6 to 8

Any one or more of the esters G, I, and J obtained in theabove-described comparative examples were employed as shown in Table 2to prepare base stocks of a lubricant (i.e., base stocks 9 to 11). Thebase stocks were tested in the same manner as in Example 1. Table 2 alsoshows the results. TABLE 2 Kinematic Ester Acid Hydroxyl viscosity ColorVolume content Base value value (mm²/s) Viscosity number resistivity (wt%) stock (mgKOH/g) (mgKOH/g) 40° C. 100° C. index (APHA) (TΩ · m) Ex. 7A (100) 1 0.01 0.4 106.9 11.14 87 68 6.2 8 B (100) 2 0.01 1 90.54 10.0289 57 5.4 9 C (100) 3 0.01 0.5 79.76 9.127 87 46 5.1 10 D (100) 4 0.010.8 75.82 8.831 87 42 4.3 11 E (100) 5 0.01 0.7 76.84 8.812 84 38 3.2 12F (100) 6 0.01 0.2 68.12 8.142 84 36 2.9 13 F (93) 7 0.01 0.6 56.247.387 90 35 2.7 K (7) 14 F (50) 8 0.01 0.3 30.87 5.126 91 31 2.0 H (25)K (25) Com. 6 G (100) 9 0.01 0.6 78.36 9.057 87 52 2.1 Ex. 7 G (20) 100.01 0.6 175.1 16.31 88 76 6.5 J (80) 8 G (7) 11 0.01 0.6 111.6 11.31 8448 2.5 I (90) J (3) Long-term Two-phase separation low-temp. temperatureFalex test test*¹ R-134a R-407C Sealed tube test Pin Pour (presence orLow High Low High Color Acid wear point absence of temp. temp temp. tempnumber value amount (° C.) crystal) (° C.) (° C.) (° C.) (° C.) (APHA)(mgKOH/g) (mg) Ex. 7 −32.5 − −10 80< 5 48   88 0.12 7 8 −35.0 − −10 80<7 58   74 0.08 8 9 −37.5 − −16 80< 3 66   60 0.10 9 10 −37.5 − −18 80<−11 80< 55 0.10 9 11 −37.5 − −17 80< −13 80< 49 0.08 10 12 −40.0 − −1880< −12 80< 47 0.09 11 13 −40.0< − −21 80< −14 80< 45 0.14 12 14 −40.0<− −30 80< −10 80< 41 0.07 13 Com. 6 −37.5 + −19 80< −15 80< 68 0.06 13Ex. 7 −32.5 − 3 80< x*² x*² 99 0.16 9 8 −25.0 + −19 80< −60 80< 63 0.1210*¹Holding at −20° C. for 1000 hours. The indication “−” denotes that noprecipitation occurs, and the indication “+” denotes that precipitationoccurs.*²Not dissolved.

As is apparent from the results in Table 2, the base stocks of alubricant (i.e., base stocks 1 to 8) in the examples had excellentstability, so that no precipitate occurred even in the long-termlow-temperature test. Moreover, these base stocks 1 to 8 have a low pourpoint, excellent compatibility with fluorocarbon refrigerants, which isindicated by the two-phase separation temperature, and lesssusceptibility to deterioration due to thermal oxidation, which isindicated by the results of the sealed tube test, and satisfy the otherperformance requirements required for base stocks of a refrigerationlubricant. Therefore, it is apparent that these are excellent base oils.In contrast, as for the base stocks of a lubricant (i.e., base stocks 9and 11) in Comparative Examples 6 and 8, precipitate occurred in thelong-term low-temperature test, so that it is found that they areinferior in low-temperature stability. As for the base stock of alubricant (i.e., base stock 10) in Comparative Example 7, no precipitateoccurred in the long-term low-temperature test, but the two-phaseseparation temperature on the low temperature side is high, so thatcompatibility with fluorocarbon refrigerants under low temperature ispoor, and also the color number after the sealed tube test is high.Thus, this base stock (i.e., base stock 10) is not sufficient forpractical use.

The refrigeration lubricant composition of the present invention hasexcellent low-temperature stability. Moreover, it has good compatibilitywith fluorocarbons, in particular, chlorine-free hydrofluorocarbons, sothat it is preferably used as a lubricant for refrigerating machinesusing a chlorine-free hydrofluorocarbon refrigerant or as a refrigerantworking fluid, obtained by mixing this lubricant with a chlorine-freehydrofluorocarbon refrigerant. Specifically, the refrigeration lubricantcomposition of the present invention and the refrigerant working fluidcontaining the lubricant and a chlorine-free hydrofluorocarbonrefrigerant can be used for air conditioning equipment such as room airconditioners and packaged air conditioners; low temperature apparatuses;industrial refrigerators; and compressors of automotive air conditionerssuch as hybrid cars and electric cars.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not limiting. The scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

1. A refrigeration lubricant composition comprising a mixed esterobtained from a mixed alcohol and a mixed carboxylic acid as a maincomponent, wherein the mixed alcohol comprises 65 to 99.95 mol % ofpentaerythritol and 0.05 to 35 mol % of dipentaerythritol, the mixedcarboxylic acid comprises 25 to 55 mol % of monocarboxylic acid having 5to 8 carbon atoms and 45 to 75 mol % of isononanoic acid, and thecomposition has a hydroxyl value of 5.0 mgKOH/g or less and an acidvalue of 0.05 mgKOH/g or less.
 2. The composition of claim 1, whereinthe mixed ester is obtained by a reaction of the mixed alcohol and themixed carboxylic acid, wherein the mixed carboxylic acid and the mixedalcohol comprise carboxylic acids and alcohols, respectively, whichsatisfy the following relation: $\begin{matrix}{1.09 \leqq \frac{\left\{ {{\sum\quad\left( {A \times B} \right)} - \left( {C \times D} \right)} \right\}}{E} \leqq 1.33} & (1)\end{matrix}$ A: number of carbon atoms in main chain of a carboxylicacid in the mixed carboxylic acid B: molar fraction of the carboxylicacid of item “A” C: number of carbon atoms in the longest side chain ofcarboxylic acids in the mixed carboxylic acid D: molar fraction ofcarboxylic acid having the longest side chain E: average number ofhydroxyl groups of each alcohol in the mixed alcohol
 3. The compositionof claim 1, wherein a kinematic viscosity of the composition at 40° C.is 30 to 150 mm²/s.
 4. The composition of any of claim 1, wherein theisononanoic acid contains 88.50 to 99.95 mol % of3,5,5-trimethylhexanoic acid.
 5. The composition of any of claim 1,wherein the composition is used for a chlorine-free hydrofluorocarbonrefrigerant.
 6. A refrigerant working fluid comprising the refrigerationlubricant composition of claim 1 and a chlorine-free hydrofluorocarbonrefrigerant.
 7. A refrigerant compression type refrigerating apparatuscomprising a compressor, a condenser, an expansion mechanism, anevaporator, and the working fluid of claim
 6. 8. The composition ofclaim 2, wherein a kinematic viscosity of the composition at 40° C. is30 to 150 mm²/s.
 9. The composition of claim 2, wherein the isononanoicacid contains 88.50 to 99.95 mol % of 3,5,5-trimethylhexanoic acid. 10.The composition of claim 3, wherein the isononanoic acid contains 88.50to 99.95 mol % of 3,5,5-trimethylhexanoic acid.
 11. The composition ofclaim 2, wherein the composition is used for a chlorine-freehydrofluorocarbon refrigerant.
 12. The composition of claim 3, whereinthe composition is used for a chlorine-free hydrofluorocarbonrefrigerant.
 13. The composition of claim 4, wherein the composition isused for a chlorine-free hydrofluorocarbon refrigerant.
 14. Arefrigerant working fluid comprising the refrigeration lubricantcomposition of claim 2 and a chlorine-free hydrofluorocarbonrefrigerant.
 15. A refrigerant working fluid comprising therefrigeration lubricant composition of claim 3 and a chlorine-freehydrofluorocarbon refrigerant.
 16. A refrigerant working fluidcomprising the refrigeration lubricant composition of claim 4 and achlorine-free hydrofluorocarbon refrigerant.
 17. A refrigerant workingfluid comprising the refrigeration lubricant composition of claim 5 anda chlorine-free hydrofluorocarbon refrigerant.